ABSTRACT: APPLYING NEW INSIGHTS TO DEVELOP VACCINE STRATEGIES THAT CIRCUMVENT AGE-ASSOCIATED IMMUNE DEFECTS With age, defects develop in naive CD4 that impair helper responses and seem to be the major factor limiting B cell responses, including long-lived IgG responses. We found that when aged mice respond to inactivated influenza A vaccine, the CD4 helper and B cell Ab responses are much enhanced by adding TLR agonist- activated dendritic cells (DC) as the antigen-presenting cells (APC) to provide optimal Ag presentation (Brahmakshatriya et. al., 2017). In studies in young mice, we found that the extent of CD4 T cell memory is dependent on the responding CD4 T cells re-engaging in a cognate interaction with APC at their effector stage, 5-8 days of their response, which we call the memory checkpoint (Bautista et al., 2017). Finally we found that the generation of T follicular helpers (TFH), critical for generating long-lived B cell Ab response also requires Ag recognition at this same checkpoint. Thus development of memory is dependent on optimal Ag presentation both initially and when effector T cells peak. However, many vaccines currently used, fail to provide the later Ag signals and in some cases they also may not provide sufficiently strong signals at either time. We will test this premise here, by vaccinating with formulations mimicking a common vaccine and then providing the signals we have defined that are required for optimal TFH, B cell response and CD4 memory. Thus, we propose that vaccine strategies that activate APC at the initiation of response and then again later, at the memory checkpoint, will synergize to enhance the response of aged CD4 T cells, and in doing so will largely circumvent their age-associated defects that hamper the generation of protective memory CD4 T cells and B cells. We will use a common vaccine, inactivated influenza, and add Ag on APC to provide optimal Ag presentation at initial vaccination and at the peak of CD4 effector response. In Aim 1, we will evaluate the impact on a) CD4 T cells effector and memory generation, on b) germinal center B cells generation and production of Ab to influenza, and on c) generation of long-lived plasma cells and B cell memory. In Aim 2 we will determine the impact of each treatment on long-term protection against lethal doses of live influenza. We predict we will show that the lack of well-activated APC initially and of persistent Ag presentation at the effector stage checkpoint limits the efficacy of inactivated vaccines for the aged and young and that by providing activated Ag/APC at both times, vaccine efficacy will be much improved. We predict the same paradigm is likely to apply to human immunization and that these basic studies will justify translation to humans.